1. Field of the Invention
The invention relates to a method for operating a pressure transducer comprising a pressure sensor, which has a measurement membrane that deflects depending on media pressure, on a base body, between which a cavity is enclosed with a reference pressure, and to a pressure transducer for process instrumentation.
2. Description of the Related Art
In industrial process control systems, a wide range of field devices for process instrumentation is employed to control processes. Measuring transducers are used to capture process variables, such as temperature, pressure, flow rate, fill level, density or gas concentration of a medium. The process sequence can be influenced by final control elements depending on captured process variables in accordance with a strategy predetermined, for example, by a control station. The invention relates, in particular, in the pressure measuring technology sector to field devices having a measuring cell which can be assigned to the class of “dry” pressure measuring cells. With this type of pressure measuring cells, no oil filling is present that transmits the pressure of a process medium to a small and sensitive pressure sensor that, for example, has a silicon membrane. In dry pressure measuring cells on the other hand, the pressure sensor has a membrane in contact with the medium which can be deflected depending on a media pressure to be measured.
WO 2013/004438 A1 discloses a dry pressure measuring cell employing capacitive capture of the deflection of the measurement membrane. The heart of the measuring cell is formed by a ceramic pressure sensor that has a flexible measurement membrane and a base body, between which a cavity is enclosed with a reference pressure. The deflection of the measurement membrane is thus dependent on a media pressure, which is present on the media side at the measurement membrane, and on the reference pressure in the cavity. Two capacitors are used to capacitively capture the deflection of the measurement membrane, where a first capacitor has (as a measuring capacitor) a measuring electrode that is arranged on the inner wall of the cavity in the central region of the base body, and where a second capacitor has (as a reference capacitor) a reference electrode which is situated in the edge region of the base body on the inner wall of the cavity. A metallization that is applied on the side of the measurement membrane that faces the cavity forms a common counter electrode for the two capacitors. It is described that in the fault-free case the capacitance of the reference capacitor is dependent on the capacitance of the measuring capacitor in accordance with an empirically learned function. For the purpose of fault diagnosis, monitoring is performed during operation of the pressure transducer to determine whether value pairs of the capacitance of the reference capacitor and the capacitance of the measuring capacitor conform to the learned functional relationship within a predefined tolerance range. If this is not the case, this is an indication of damage to the measuring cell. Damage can thereby primarily be recognized, which results in a change in the flexural line of the measurement membrane. Possible causes mentioned are a malfunction of the fixed connection at the joint between measurement membrane and base body or radial compressive offset stresses or yield stresses introduced into the measurement membrane by way of the mounting of the pressure sensor. Changes in the composition of the medium situated in the cavity can however not be reliably recognized.
Under harsh process conditions, such as those existing in the paper industry, or in the case of sudden temperature changes due to cleaning or sterilization in the food industry, tears or breaks may occur that particularly affect the membrane, as a result of which process medium penetrates into the cavity of the ceramic pressure sensor. This can lead to a malfunction of the pressure transducer. The fact that a pressure transducer has failed on account of penetrating process medium can, for example, be recognized as a result of detecting a short-circuit between capacitor electrodes of the pressure sensor. It is, however, desirable that an impending malfunction is recognized as early as possible in advance so that maintenance personnel are able to plan appropriate measures in advance, such as the timely replacement of the pressure transducer, and execute the measures at a suitable point in time such that unforeseen downtime of an industrial process control system in which the pressure transducer is employed can be avoided.